ABSTRACT For the competing renewal of our R01NS34467-19 project, we propose to continue collaborative studies between the Zlokovic and the Holtzman labs aimed at understanding at the cellular, molecular and systems level, and in allele-specific and gender-specific manner apolipoprotein E (apoE) effects on the cerebrovascular system, specifically, 1) how they contribute to Alzheimer?s disease (AD) neurovascular dysfunction, neurodegenerative, amyloid-? (A?) and tau disorders; and 2) how targeting apoE-low density lipoprotein receptor-related protein 1 (LRP1) interactions on different cerebrovascular cell types can mitigate and/or block cerebrovascular and brain damage causing dementia in AD. We will focus on APOE4, the strongest genetic risk factor for AD, and APOE3 that carries a significantly lower risk for AD. Neurovascular apoE is produced by astrocytes, pericytes, and vascular smooth muscle cells (VSMCs). Based on published and our pilot data, we hypothesize that apoE isoform-specific interactions with LRP1 on brain vascular cells provide a pathogenic link to apoE-related cerebrovascular disorder by controlling in an apoE-isoform-specific manner: 1) cyclophilin A (CypA)-matrix- metalloproteinase 9 (MMP9) pathway which disrupts blood-brain barrier (BBB) and arterial blood vessels; and 2) Ab and tau clearance on capillary and arterial vessels. Specifically, the apoE4 pathophysiological cerebrovascular effects accelerate cerebrovascular, neurodegenerative, Ab and tau disorders, whereas apoE3 has less disruptive effects. ApoE allele-specific effects on BBB integrity, vascular and neuronal function and structure will be studied in new APOE knockinflox/flox E3F and E4F mice with apoE cell-specific deletion from astrocytes, pericytes, or VSMCs (AIM 1); and, in the context of Ab and tau pathology, in APP/PS1-21 mice (AIM 2) and P301S tau mice (AIM 3), respectively, crossed with our new E3F and E4F mice with apoE cell-specific deletion from astrocytes, pericytes, or VSMCs. We will therapeutically target LRP1 and CypA-MMP9 pathway with intravenous cell-specific adeno-associated virus (AAV)-mediated delivery of LRP1 minigene to endothelium and pericytes, and CypA inhibition with Debio 025. Understanding the effects of astrocyte-derived, pericyte- derived and/or VSMC-derived apoE on cerebrovascular and neuronal functions, Ab and tau clearance and pathology, in an allele-specific and gender-specific manner, will advance our understanding of the pathogenesis and treatment of AD neurovascular dysfunction, neurodegeneration, Ab and tau disorders. If successful, this could lead to new therapeutic approaches targeting LRP1 and/or CypA-MMP9 pathway in different brain vascular cells to mitigate and/or block apoE?s cerebrovascular and brain damage causing dementia in AD.